Can't locate External Hard Drive in a MAC

Hello... I can not locate my external hard drive in order to back up my laptop. I have a Leopard upgrade, but read that I should back up my computer. (which I have NEVER done, and I've had it for almost 3 years) However, when following the instructions I found online, as to how to back up, I can not locate the external hard drive anywhere. It does not show up in disk utilities either. ????

Thank you! Well... It was a cord issue. It is now locating the hard drive, and I can select it in Disk Utility for back-up. HOWEVER, now I get and error saying that it is unable to backup because the "resource is busy!" argh!! I've looked online to find help... no luck! I tried the "unmount" thing.. then I couldn't even select it! Any help would be GREATLY appreciated!

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